1. Field of the Invention
The invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium used in a high-density hard disk drive.
2. Description of the Prior Art
High-density hard disk drives comprising a plurality of magnetic recording media are currently widely used in computer systems for recording large amounts of data. In recent years, considerable effort has been made in achieving higher storage density and coercivity through the development of magnetic recording media using magnetic thin films. Key terms for factors affecting the storage density of magnetic recording media are defined as follows:
(a) Coercivity (Hc): the magnetic field required to reduce remanence magnetic flux to zero. A higher coercivity is associated with a higher information storage density by allowing adjacent recording bits to be placed more closely without mutual cancellation. Most material used in the industry today have an Hc greater than 2200 Oersteds (Oe).
(b) Orientation Ratio: the ratio of the coercivity of the magnetic track in the circumferential direction to that in the radial direction. An orientation ratio close to 1 indicates that the magnetic recording has a very stable signal output as its magnetic properties are less affected by the surface texture of the magnetic recording medium.
(c) Signal-to-Noise Ratio (SNR): defined as 20*log [Signal Voltage/Noise Voltage]. A high SNR is associated with a high bit density to be read with a given degree of reliability since more signals can be detected in a low noise reading operation setting.
(d) Off Track Capability (OTC): defined as the capability of a read/write head to be shifted away from a track while information imprinted on the track is still being read.
(e) Overwrite capability (Ow): defined as 20*log [Residual LFTAA/Original LFTAA]. Overwrite capability is the effectiveness of erasing a signal read at one frequency by a higher frequency signal and provides a measure of a remaining residual signal after the old signal is overwritten by a new signal. The more negative the value of Ow the better the overwrite capability. Generally, the value is required to be less than -26 dB.
(f) Positive, Negative Modulation (PMOD, NMOD): PMOD is defined as the continuous signal present in a track above a designated percentage whereas NMOD is defined as the continuous signal present in a track below a designated percentage.
Currently, magnetic thin films in magnetic recording media of hard disk drives often use CoCrPtTa alloy/Cr, CoCrPtTa alloy/CrV alloy, or CoCrPt alloy/NiAl alloy as their magnetic layer/sublayer structures. The CoCrPtTa/Cr and CoCrPtTa/CrV alloy magnetic thin films and their off track capability (OTC) need improvement as do their orientation ratios which, at 1.2, is far removed from the most optimal value of 1. The CoCrPtTa/CrV magnetic thin film and the CoCrPt alloy/NiAl alloy magnetic thin film can generate high coercivity, however, both come with a price. To achieve high coercivity, the CoCrPtTa/CrV magnetic thin film generates high levels of noise and the CoCrPt alloy/NiAl alloy magnetic thin film requires the thickness of its magnetic layer to be over 1000 .ANG. while the NiAl alloy base layer must be between 400 .ANG..about.500. Such rigid requirements in the specific thickness of the various parts may cause implementation problems when used in some industrial applications.